Ambulatory cardiac diagnostic units having means for inhibiting pacemaker response

ABSTRACT

The diagnostic unit 1 consists of a microprocessor-based control section 5, a data memory 6 and a pacemaker 7. The control section 5 monitors heart action using the intracardiac electrogram and a second signal representative of intracardiac pressure. It detects events of interest to clinicians including bradycardia, tachycardia, cardiac pauses, pressure pauses, interference and pacing pauses, and records the numbers of each type of event. Selective recordings of the waveforms of the sensed signals are made in memory within the unit whenever an event of interest is detected. The unit is worn by the patient for periods of up to three weeks, and the parameters of the events to be sensed can be programmed into the unit by the clinician using a computer 4. The same computer 4 is used to extract and display the recorded data.

This invention relates to ambulatory cardiac diagnostic units, that isto units which may be worn by a patient and which serve to monitor thepatient's heart with a view to providing data which may be used todiagnose heart malfunction.

It is known for patients having certain heart disorders to be fittedwith a pacemaker which may be implanted by surgery and which provideselectrical outputs for pacing beating of the heart. It is also known tomonitor the electrocardiogram of a patient over a the period of a day byusing a 24-hour tape loop recording device which may be worn by the user(N. J. Holter, "New Method for Heart Studies: ContinuousElectrocardiography of Active Subjects", Science 134:1214, 1961).

However, the shortcomings of such Holter monitoring has been apparent inmany clinical situations for some time, and there has been a need forsome form of recording pacemaker. The demand for such a recordingpacemaker would be considerable, particularly for the diagnosis of heartdisease causing infrequent disturbances and unexplained syncope (D. B.Shaw, C. A. Kekwick, D. Veale, T. W. Whistance, "Unexplained Syncope",Pace July 6, 1983). Current data emphasises the difficulty in diagnosisof heart disease under such conditions, even after complex studies inhospital. Most pacing centres which do not have a special interest insinus node disease do not have the time or facilities to undertakeprolonged investigations. In the absence of any clear evidence of anymajor sinus node function disturbance the patient's symptoms arecommonly labelled as "non-cardiac". However the following publicationssuggest that further follow-up investigations would indicate thatpotential benefit would result from pacing with an appropriate unit: R.A. Winkle "Long-term electrocardiographic and event recorders for thediagnosis and treatment of cardiac arrhythmias", Circulation(Supplement) 75: III-53, 1987; J. T. Bigger "Perspectives on long-termrecording and monitoring", Circulation (Supplement) 75: III-58, 1987; D.B. Shaw, T. W. Whistance, "Clever Pacemakers", Hospital Update, November1986. U.S. Pat. Nos. 4,183,354, 4,250,888, 4,363,397 and 4,513,743disclose various forms of diagnostic recorder but none of these matchesthe criteria which are believed to be of importance in providingsatisfactory diagnosis under a variety of conditions.

It is an object of the invention to provide a generally improved form ofambulatory cardiac diagnostic unit.

According to the present invention there is provided an ambulatorycardiac diagnostic unit comprising recording means for recording dataindicative of cardiac function for subsequent analysis, the recordingmeans being adapted to record data representative of electrical activityof the heart measured over a cardiac event monitoring period, andcontrol means for controlling recording over said cardiac eventmonitoring period in response to sensing of a cardiac event,characterised in that the unit further comprises pacing means adapted toinitiate pacing of the heart under control of said control means suchthat following sensing of a heart malfunctiuon pacing of the heart isinitiated after recording of data indicative of a cardiac event hastaken place over said cardiac event monitoring period.

Preferably the control means is adapted to control the pacing means soas to give demand pacing for a predetermined duration following saidcardiac event monitoring period.

The recording means may also be adapted to record data indicative of asecond cardiac function, such as intracardiac pressure (or rate ofchange of pressure) or intracardiac impedance.

Preferably both signals are used in combination to provide reliabledetection of heartbeats. Furthermore the unit may be adapted to detectwhether "capture" has occurred during pacing.

Conveniently the signals are sensed using a single intracardiaccatheter. Furthermore the unit may be adapted to measure the intervalbetween sensed beats, and optionally to store a histogram of the timeintervals between successive beats. This time interval may be used todetect bradycardia or tachycardia. Furthermore the time interval may beused to detect a pause in pressure beats.

Additionally the unit may be adapted to measure the duration since thelast detected heartbeat, for example so as to detect asystole.Furthermore the sensed signals may be used to detect electricalinterference.

The unit may be adapted to initiate pacing in response to prolongedbradycardia, pause events or interference.

In order that the invention may be more fully understood, a preferredembodiment of the invention will now be described, by way of example,with reference to the accompanying drawings, in which:

FIG. 1 shows an overall block diagram of the unit;

FIG. 2 shows a block diagram of a control section of the unit.

The ambulatory cardiac diagnostic unit which will now be described withreference to the drawings is intended to simultaneously measure, processand selectively record both a first signal being the intracardiacelectrogram and a second signal representative of intracardiac pressure.The unit also detects and records certain defined cardiac events forsubsequent playback. The unit may be worn externally by the patient forrelatively short periods, for example of up to three weeks, although theeventual objective is an implantable device which may be wornindefinitely.

The cardiac events which are detectable by the unit may be defined asfollows:

Bradycardia Event - Bradycardia is said to occur if the averageheartrate over 1-8 interbeat periods (programmable) is at or below acertain heartrate. The heartrate at which bradycardia is detected isprogrammable in the range of 25-45 bpm.

Pause on the Intracardiac Electrogram Channel - A pause is said to occuron the electrical channel if a heartbeat is not detected for a certainlength of time. The minimum time span for the definition of a pause isprogrammable.

Pacing Pause - It is known that on occasions the heart does not beatalthough it has been paced by a pacemaker (i.e. capture has notoccurred). This event will be called a pacing pause.

Tachycardia Event - Tachycardia is said to exist when the averageheartrate over a selected number of interbeat periods (3-20) is at orabove the selected heartrate (100-200 bpm). Both the number of interbeatperiods and the selected heartrate are programmable.

End of Tachycardia - The end of tachycardia is defined such that theheartrate must remain below the selected heartrate for a preset numberof consecutive interbeat periods. The number of interbeat periods canalso be set between 3 and 20. Consecutive beats are used instead ofaveraging over a certain number of beats (as with the detection of thestart of tachycardia). This ensures that if the heartrate is very nearthe rate for tachycardia and drifts above and below it, there is lesschance of filling up the memory allocated for tachycardia with separateportions of the same tachycardia.

Pressure Pause - A pressure pause is defined as the occurrence of anelectrical heartbeat without a corresponding pressure beat. In practicean electrical beat from a contraction of the heart is detected beforethe corresponding pressure beat. Therefore for detection a pressurepause is defined as two consecutive electrical beats without a pressurebeat between them.

Interference - The detection of interference on the electrical channelrelies on the detection of interference spikes. These spikes must passan amplitude threshold and occur at greater than a preset frequency. Theamplitude threshold programmable and the minimum frequency forinterference is 10 Hz.

FIG. 1 shows the diagnostic unit 1 connected by leads to the heart 2. Italso shows a hand-held battery operated key 3 and a computer 4 to whichthe unit 1 may be selectively connected. Parameters are transferred fromthe computer 4 to the diagnostic unit 1 prior to connecting thediagnostic unit 1 to the transducer catheter (not shown), that is to thepatient's heart. The computer 4 is disconnected from the diagnostic unit1 before the unit 1 is attached to the catheter. Monitoring is thenstarted by sending a signal to the unit 1 from the hand-held key 3 whichis temporarily connected to the unit 1 for this purpose. The same key 3is also used to stop monitoring prior to extraction of the recorded datafrom the unit 1 to the computer 4.

The diagnostic unit 1 consists of a control section 5, a data memory 6forming part of the control section 5, and an adapted commercialpacemaker 7, together with transducers and batteries (not shown).Optoisolators 8 are used to provide electrical isolation of the serialcommunication port 9. The terminals A and B of the port 9 may beconnected either to the terminals A', B' of the computer 4 or to theterminals A", B" of the key 3.

The pacemaker 7 used is a demand pacemaker, that is it only paces if thetime from a spontaneous beat before a further spontaneous beat isdetected exceeds a preset time. The pacemaker 7 is partially controlledby the control section 5 in that the timing mechanism (or clock) of thepacemaker 7 can be switched on or off by the control section 5 by meansof a clock on/off signal line 10. While the clock is switched on pacingcan occur, but pacing cannot occur while the clock is switched off.Detection of intracardiac electrogram beats is carried out by thepacemaker 7 regardless of whether the clock is on or off.

An intracardiac electrogram signal is supplied both to the pacemaker 7and to the control section 5 by a signal line 11, and a second heartsignal representative of intracardiac pressure is supplied by a signalline 12. The control section 5 detects intracardiac electrogram beatsindependently of the pacemaker 7 so as to allow pacing pauses to bedetected. When a spontaneous beat is detected by the pacemaker 7 itsends a signal to the control section 5 by way of a sense signal line13. Furthermore the pacemaker 7 also sends a signal to the controlsection 5 by way of a pace initiate signal line 14 when a pacing pulseis generated. These signals allow interbeat periods to be calculated bythe control section 5.

Referring to FIG. 2 the control section 5 comprises two microprocessors20 and 21 sharing a common memory consisting of an 128 K byte RAM module22 and a 2 K×8 byte RAM 23, and an address, data and control bus 24. Theuse of two microprocessors sharing common memory allows the separationof the data acquisition and data processing tasks. The twomicroprocessors 20 and 21 are a 65C112 and a 65C102. By the use of thetechniques described in "A low-cost high performance 64 K shared memorysystem", J.M.K. Horwood and J. Baker, J. of Microcomputer Applications,1985, 8 both microcomputers retain full access to all the system memoryand input/output devices. Virtually no additional logic is required tofacilitate this.

2 K bytes of static RAM are provided for purposes other than recordeddata storage. This includes the stacks for both microprocessors. Themain data storage is 128 K bytes of static RAM, although provision ismade for up to 256 K bytes to allow for system expansion.

In operation the control section 5 continually records the intracardiacelectrogram and the second signal indicative of intracardiac pressure. Asampling rate of 400 Hz is used for the intracardiac electrogram,whereas a sampling rate of 100 Hz is used for the second signal.However, in the absence of a detected cardiac event, the recorded datais overwritten by newly recorded data after a predetermined interval oftime. In the described embodiment 8 K bytes of data are stored for eachrecorded waveform, this being equivalent to about sixteen seconds ofrecording. On detection of a cardiac event the control unit produces arecording which is not subsequently overwritten and which is dividedinto two parts, that is a part recorded before detection of the eventand a part recorded after detection of the event. For example twelveseconds of recording may be carried out before the event for bothBradycardia and intracardiac electrogram pauses, leaving four seconds ofrecording after the detection of the event. For tachycardia, pressurespauses, pacing pauses and interference, eight seconds of recording aretaken before and after the detection of the event. Sixteen suchrecordings may be made in sixteen memory blocks of the module 22, eachof which is of 8 K bytes.

When monitoring begins, the first sample value is stored at the firstaddress of the first memory block, and at the same time a code is storedin a register which indicates which block is to be accessed. A pointerindicates at which memory location the sample value is to be stored, andthis pointer is incremented after each sample is stored. When thepointer reaches the end of the first memory block (after about sixteenseconds) it returns to the beginning of this memory block and overwritesthe previously recorded data. Thus the memory block always contains themost recent sixteen seconds of recorded waveforms.

When an event waveform is to be recorded on detection of a cardiacevent, the value of the pointer is recorded. This gives the address ofthe start of the event, that is the point at which the event isdetected, and the sample values are recorded in the memory block for afurther preset number of bytes. A part of the previously recorded datais not overwritten, and this part then constitutes the recordingimmediately preceding the event.

When the recording of the event is complete, the pointer moves to thestart of the next memory block, and this next memory block is thencontinuously overwritten until the next event waveform is to berecorded. This process is continued until all sixteen event waveformshave been recorded. At this point no further sample values are stored inthe memory.

The programmes and fixed data for both microprocessors are contained ina single 32 K byte CMOS EPROM which appears in the address map as twobanks of 16 K bytes each of which is only accessed by a singlemicroprocessor.

An analogue to digital convertor 25 is used for the intracardiacelectrogram and pressure channels. This convertor 25 may be constitutedby a single ADC0844 which incorporates a 4-channel multiplexer. Thecontrol section 5 also includes a clock unit 26 and a reset unit 27.

Furthermore the connection of the control section 5 to the pacemaker 7is effected by way of an input/output circuit 28 and a safety overrideunit 29. In the event of control section failure the safety overrideunit 29 operates to prevent the clock of the pacemaker remaining offwhen no spontaneous beats have been detected for longer than a certainlength of time.

By way of further explanation of the function of the control mechanismsome general definitions of terminology used in this specification willnow be given.

Dormant Periods - Dormant periods are introduced for the comfort andsafety of the patient. In order to record the waveform of a bradycardiaevent or a pause on the intracardiac electrogram channel the pacemakeris inhibited for a short time (i.e. it is not allowed to pace). Dormantperiods ensure that there is at least a preset time between successiveinhibitions of the pacemaker so that unpleasant symptoms that may beexperienced by the patient are not allowed to persist.

After the detection of a bradycardia or pause on the electrical channel,the control unit enters a dormant period during which the pacemaker isallowed to function normally. This means that no bradycardia or pauseevents should occur but monitoring of the other events and updating ofthe histogram can continue. The dormant period is programmable in therange 0-10 minutes.

There is also a further dormant period which is invoked if more than onebradycardia or pause on the electrical channel is detected within onehour. If more than one of these events does occur in an hour one of twooptions may be followed. The option to be followed is programmable.

Option 1: The pacemaker is allowed to operate normally for the remainingrecording time.

Option 2: The pacemaker is allowed to operate normally for a presetlength of time (0-11.5 hours).

The T-wave - The T-wave constitutes that portion of the intracardiacelectrogram which immediately follows a beat and is characteristic ofthe repolarisation of the heart. It is undesirable to pace on theT-wave.

Pacemaker refractory period - Once the pacemaker has detected a beat itenters a short refractory period during which it does not look for theoccurrence of a beat. This is to ensure that the T-wave (seen at the endof the beat) is not mistaken for the occurrence of another beat.

The above described diagnostic unit has a number of functions inoperation, and these may be summarised as follows:

(i) Monitoring the intracardiac electrogram and second signal usingtransducers mounted on a catheter positioned in a chamber of the heart.

(ii) Detecting electrical and pressure heartbeats. (Other time varyingsignals which indicate the function of the heart can be used).

(iii) Detecting and counting events. The events monitored arebradycardia, interference, tachycardia, pressure pauses, pauses on theelectrical channel and pacing pauses. Each time one of these eventsoccurs the relevant event counter is updated.

(iv) Allowing variable definitions of the events named above. The exactdefinitions of the events are set up on the computer and the parametersare transferred to the diagnostic unit prior to the start of monitoring.

(v) Recording the intracardiac electrogram and second signal of sixteenevents. The type and time of these events are recorded, as are the timesof the start and finish of any tachycardia or interference.

(vi) A histogram of interbeat periods is built up continuously, exceptwhen interference is present. There are nine counters with programmableranges. During interference on the intracardiac electrogram channelmonitoring of all other events ceases.

At the start of monitoring or on completion of the dormant periods thepacemaker clock is switched off. This is always done at the end of itsrefractory period (during which no beats can be detected). The pacemakerclock is always switched on during the dormant periods or on thedetection of bradycardia. This is done immediately after the end of arefractory period. The pacemaker is then capable of sensing anintracardiac electrogram beat immediately and there is no chance ofmissing the detection of a beat or pacing the heart on the T-wave. Theclock is also switched on when a pause or interference is detected. Inthese cases the clock is switched on immediately as there are nospontaneous or paced beats detected. In this case of interference theclock remains on until the end of interference is detected.

The definitions of the events and the recording parameters are set up onthe computer prior to the start of monitoring. These parameters are thentransferred to the diagnostic unit so that monitoring can commence. Theprogrammable parameters are listed below:

1. The heartrate defining bradycardia.

2. The number of interbeat periods that are averaged in the detection ofbradycardia.

3. The duration of time that must elapse from the detection of aspontaneous beat to define the occurrence of a pause.

4. The duration of the dormant period associated with the detection of abradycardia or intracardiac electrogram pause event.

5. The option for a dormant period associated with the detection of morethan one pause or bradycardia in an hour and the duration of the dormantperiod if the shorter option is chosen.

6. The number of interbeat periods that are averaged in the detection oftachycardia.

7. The heartrate defining tachycardia.

8. The number of interbeat periods that are averaged in the detection ofthe end of tachycardia.

9. The heartrate defining the end of tachycardia.

10. The ranges of the histogram counters.

11. The number of bradycardia event waveforms that can be recorded.

12. The number of tachycardia event waveforms that can be recorded.

13. The number of intracardiac electrogram pause waveforms that can berecorded.

14. The number of pressure pause waveforms that can be recorded.

15. The number of interference event waveforms that can be recorded.

16. The number of pacing pause waveforms that can be recorded.

17. The number of pressure pause waveforms to be recorded.

18. The signal amplitude threshold for interference.

19. The pressure signal amplitude threshold to define a pressure beat.

20. The duration for which the pressure signal must remain above theamplitude threshold to define a pressure beat.

At the end of monitoring the diagnostic unit

is disconnected from the patient and attached to the computer. Therecorded data is then transferred from the diagnostic unit to thecomputer and stored for future reference.

We claim:
 1. An ambulatory cardiac diagnostic unit comprising pacingmeans for pacing the heart, sensing means for sensing electricalactivity of the heart, detection means connected to said sensing meansfor monitoring data outputted by the sensing means which isrepresentative of the electrical activity of the heart and for detectingfrom said data the occurrence of a heart malfunction, recording meansconnected to the sensing means and the detection means for recording, inresponse to detection of a heart malfunction, said data over a cardiacevent monitoring period which includes periods both before and afterdetection of the heart malfunction, and control means for (a)controlling recording by said recording means of said data over saidcardiac event monitoring period, and (b) controlling said pacing meansso as to inhibit pacing of the heart following detection of a heartmalfunction and until recording of said data over said cardiac eventmonitoring period has been completed by said recording means.
 2. Anambulatory cardiac diagnostic unit according to claim 1, wherein saidcontrol means is adapted to control said pacing means so as to givedemand pacing for a predetermined duration following said cardiac eventmonitoring period.
 3. An ambulatory cardiac diagnostic unit according toclaim 1, wherein the recording means is adapted to continuously recorddata with the previously recorded data being overwritten by newlyrecorded data after a predetermined interval of time, and the controlmeans is adapted to control recording in response to detection of aheart malfunction so that an initial part of the previously recordeddata is overwritten leaving recorded data representative of theelectrical activity of the heart over said cardiac event monitoringperiod.
 4. An ambulatory cardiac diagnostic unit according to claim 1,wherein further sensing means are provided for sensing a cardiacfunction, and the recording means is also adapted to record dataindicative of said cardiac function.
 5. An ambulatory cardiac diagnosticunit according to claim 4, wherein said cardiac function is intracardiacpressure.
 6. An ambulatory cardiac diagnostic unit according to claim 1,wherein the recording means is adapted to record data over a pluralityof cardiac event monitoring periods.
 7. An ambulatory cardiac diagnosticunit according to claim 1, wherein the detection means is adapted tomonitor the interbeat periods between successive heartbeats and toindicate the occurrence of a heart malfunction in response to suchmonitoring.
 8. An ambulatory cardiac diagnostic unit according to claim1, wherein the detection means is adapted to compare data indicative ofmonitored heart electrical activity with preset values indicative ofacceptable heart performance.
 9. An ambulatory cardiac diagnostic unitaccording to claim 8, wherein the detection means records the number ofheart malfunctions detected and their timings.
 10. An ambulatory cardiacdiagnostic unit according to claim 1, wherein it further comprisesinterference detection means for inhibiting monitoring in the event ofelectrical interference.